Method and structure for improving performance and storage density in a data storage device

ABSTRACT

A data storage device with improved data storage densities, coupled with lower hard error and write-inhibit events is described. A feed-forward write inhibit (FFWI) method enables data tracks to be written more densely. Alternatively, the FFWI method may reduce the hard error and write inhibit events to improve data storage performance. A concept of virtual tracks enables the FFWI method to be applied to the writing of circular data tracks with non-circular servo tracks, or to the writing of non-circular data tracks with PES data from circular servo tracks—in both cases, improvements to performance and/or storage densities are enabled. The FFWI method may also be applied to the case of both non-circular servo and data tracks.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to data storage devices, and in particularto data storage devices with various data storage regions havingdifferent optimization requirements, such as write inhibit rates, harderror rates, and data storage densities.

BACKGROUND OF THE INVENTION

The present invention relates to data storage devices employing rotatingdata storage media. As described in U.S. Pat. No. 7,394,607, thelocation of the write head relative to the desired location of the trackis the position error signal (PES), which is generated during passage ofthe read head over a “servo burst region” contained within a servo zone(also termed a “servo spoke”). Typical disks may have more than 250approximately radial servo zones, thereby providing PES values roughlyevery 1° of rotation of the disk medium. U.S. Pat. Publication No.2011/0188149A1 is an example of the use of servo bursts for generationof PES values.

U.S. Pat. No. 6,111,714 illustrates the use of adaptive control of thewrite inhibit signal as a function of the measured velocity of the writehead, as may occur during data storage device operation in a operationalvibration or shock environment, as may be the case for a computerplaying loud audios or movies. The benefits of reducing the number ofwrite inhibit events in such an environment are demonstrated in U.S.Pat. No. 7,633,698B2, which discusses the use of an accelerometer todetect the acceleration of the data storage device, enabling adifferentiation between vibratory and one-time shock events. Inparticular, when the disk drive is not subject to vibration, the writeinhibit threshold may be lowered to enable more sensitive detection ofone-time shock events. Under vibratory conditions, the write inhibitthreshold may be increased to a more normal level, thereby avoiding anexcessive number of WI events. Operational shock may result in large PESvalues over short time periods during which it may be preferred toprevent write or read events, thereby avoiding writing outside thedesired track regions, and possibly even “head crashes”.

FIG. 1 is a schematic diagram 100 of two neighboring tracks 102 and 104in a disk storage device with write inhibit based on a fixed PES range.The track center 106 for track 102 is shown as a curved line,representing a sequence of position error signals (PES) generated eachtime the read head passes over a servo burst region while writing track102. The track center 108 for track 104 differs from the track center106 for track 102 as shown. Dashed line 110 represents the maximumpositive PES value for writing track 102—any PES values larger than this(representing cases where track center 106 extends to the right of line110), will induce a write inhibit event for track 102. The distance 112between track center 102 and PES limit 110 is set equal to L (see FIG.2). Similarly, the PES limits when writing track 104 are shown as dashedlines 114 and 118. Anytime track center 108 extends to the left of line114, or to the right of line 118, a write inhibit event will betriggered. Spacings 116 and 120 are both equal to “L”. The inter-trackspacing 122 is described in FIG. 2.

FIG. 2 is a schematic graph 200 of the probabilities of various PESvalues 250 and 252 for two neighboring tracks in a data storage deviceemploying shingled method recording (SMR) without feed-forward (FF) asillustrated in FIG. 1. The desired center 202 of track 1 corresponds totrack 102 in FIG. 1, while the desired center 204 of track 2 correspondsto track 104. The vertical axis in FIG. 2 corresponds to the probabilityof the PES having the values shown along the horizontal axis 270.Experimental measurements confirm that we can assume that the PES valuesare roughly normally-distributed, falling on the two standard deviationcurves shown centered on the track centers 202 and 204. From thespecifications of the recording system, a minimum allowable trackcenter-to-center spacing, W, is determined. To ensure data integrity, awrite inhibit (WI) condition is imposed whenever the PES value exceedslimits of ±L, thus the write-inhibit criteria are (on a sector-by-sectorbasis):

PES(k)≧L, and

PES(k≦)≦−L,

where k=the track number (either 1 or 2 in this example). Each of theseevents (which are obviously mutually-exclusive) occurs with aprobability of Q(L/σ), where the Q-function is the tail probability:

Q(x)=0.5−0.5 erf(x/√2),

and erf is the error function. Thus the total probability of one or theother event occurring is clearly 2 Q(L/σ).

For proper data storage device performance, we require that 2Q(L/σ)˜10⁻³, which determines the value for L in units of T. Since inpractice L≅0.13 T, we can determine a value for σ (also in units of T)by solving for σ in this equation:

2 Q(L/σ)=10⁻³

which gives a value for the standard deviation σ≅0.04 T (the factor of“2” arises due to the possibility for both positive and negative headexcursions). Thus a PES value of 0.13 T represents a deviation of thehead from the desired track position of roughly 0.13/0.04=3.25 standarddeviations. As shown in FIG. 2, an additional margin, E, is added toaccount for PES noise so that the probability of a head excursion of L+Eis approximately 10⁻⁶:

2 Q((L+E)/σ)˜10⁻⁶.

In practice, values of E of roughly 0.05 T are used. Typical values of Tmay be around 17 nm with track widths of 7 nm. Thus we now can determinea value for “W” in units of T, since T=W+2 L+2 E:

W=T−2 L−2 E=T−2(0.13 T)−2 (0.05 T)=0.64 T.

Clearly this method for determining T may be somewhat conservative,since in all cases a worst-case potential value for the PES values ofneighboring tracks is used to determine T and W. As FIG. 1 illustrates,if track center 106 happens to be to the left of track 102, then thespacing between track center 108 and track 106 will exceed the minimumallowable spacing W, resulting in needlessly large spacings between thewritten tracks. If track center 106 is at location 260 in FIG. 2 at thesame time as track center 108 is at location 262 in FIG. 2, then thetracks will be at their minimum allowable spacing of W—in essentiallyall other situations, the actual spacing between track centers 106 and108 will be larger than W, indicating sub-optimal (too low) storagedensities (tracks per inch or TPI).

Thus it would be desirable to account for the actual track center of aneighboring track when writing the next track, especially when employingshingled writing, in order to reduce the amount of time that theinter-track spacing exceeds the minimum allowable value of W.

There are three parameters which may be used to characterize theperformance of data storage devices:

-   1) The number of write inhibit (WI) hits—we would like to minimize    this, all other parameters being constant,-   2) The number of hard errors (HE)—we would also like to minimize    this, again all other parameters being constant, and-   3) The data storage density (tracks per inch, TPI)—we would like to    maximize this, all other parameters being constant.

For different regions of the disk medium, different overall optimizationstrategies may be desirable:

-   1) In E-regions (temporary cache regions), improving performance may    be preferred, possibly at the expense of data storage density. This    is because in these regions, the data will eventually be moved to an    I-region, but for the moment, this data is being stored at the    maximum rate possible, thus minimizing the number of WI and HE    events is preferred over maximizing the TPI.-   2) In I-regions (final “home” or destination regions), improving the    areal data storage density (TPI) may be preferred. For these    regions, more dense storage is preferred since I-regions are written    whenever possible (rewriting data from E-regions) or when large    sequential writing is in progress.

Thus it would be advantageous to provide a method for feed forward writeinhibit (FFWI) control which optimizes the writing strategy for both I-and E-regions, possibly with differing optimization strategies. It wouldalso be advantageous to provide a method for write inhibit control whichimproves the performance of a disk drive in an operational vibrationenvironment.

SUMMARY OF THE INVENTION

Aspects of the present invention provide a method for improved datastorage in a hard disk drive or other data storage device employing arotating medium. In some embodiments, an improved write inhibit methodemploys a feed-forward method which takes into account the actuallocation of a neighboring data track to reduce the averagetrack-to-track spacing. Advantages of the method are improved datastorage density (tracks per inch, TPI), and reduced hard error and writeinhibit hits to the writing performance.

In some embodiments, virtual tracks enable improved storage densities,while lowering hard error and write inhibit events. Both conditions ofcircular servo tracks with non-circular data tracks, as well asnon-circular servo tracks and circular data tracks, may be addressedthrough the virtual track concept.

In further embodiments, improved performance in an operational vibrationenvironment is enabled through the feed forward write inhibit method,with trade-offs between performance and storage density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a disk storage device with writeinhibit based on a fixed PES range;

FIG. 2 is a schematic graph of the probabilities of various PES valuesfor two neighboring tracks as illustrated in FIG. 1;

FIG. 3 is a schematic diagram of a disk storage device with feed forwardwrite inhibit according to the present invention;

FIG. 4 is a schematic graph of the probabilities of various PES valuesfor two neighboring tracks according to the invention as illustrated inFIG. 3;

FIG. 5 is a graph characterizing data storage device operation accordingto the invention for writing in a temporary cache region;

FIG. 6 is a graph characterizing data storage device operation accordingto the invention for writing in a “home” or destination region;

FIG. 7 is a graph characterizing data storage device operation accordingto the invention for writing in a temporary cache region under 100%volume pink noise conditions;

FIG. 8 is a graph characterizing data storage device operation accordingto the invention for writing in a “home” or destination region under100% volume pink noise conditions;

FIG. 9 is a schematic diagram of two neighboring tracks, showing datasqueeze;

FIG. 10 is a schematic diagram of two neighboring tracks in a datastorage device according to the invention, showing a proposed solutionto the data squeeze problem illustrated in FIG. 9;

FIG. 11 is a schematic diagram of non-circular virtual data tracksaccording to the invention;

FIG. 12 is a schematic diagram of circular virtual data tracks accordingto the invention;

FIG. 13 is a schematic diagram of two neighboring virtual tracks,illustrating polynomial or harmonic fits to the PES measurements;

FIG. 14 is a flowchart for implementing the feed forward write inhibitmethod according to the present invention;

FIG. 15 is a schematic diagram of a data storage system according to thepresent invention;

DETAILED DESCRIPTION OF ALTERNATIVE EMBODIMENTS

Embodiments of the invention can provide one or more advantages overwrite inhibit methods employing fixed PES limits. Not all embodimentsmay provide all the benefits.

Feed Forward Write Inhibit (FFWI) Strategy According to the Invention

FIG. 3 is a schematic diagram 300 of two neighboring tracks 302 and 304in a disk storage device with feed forward write inhibit (FFWI)according to the present invention. The track center 306 for track 302is shown as a curved line, representing a sequence of position errorsignals (PES) generated each time the read head passes over a servoburst region while writing track 302. The track center 308 for track 304differs from the track center 306 for track 302 as shown. Dashed line310 represents the maximum positive PES value for writing track 302—anyPES values larger than this (representing cases where track center 306extends to the right of line 310), will induce a write inhibit event fortrack 302. The distance 312 between track center 302 and PES limit 310is generally set equal to L.

A key difference in the present invention is the method for setting theleft-side (negative PES) write-inhibit threshold, dashed line 314. Ascan be seen in FIG. 3, line 314 has the same shape as track center 306but offset to the right a distance 324, at a spatial resolution alongthe tracks corresponding to the spacing of PES measurements along tracks302 and 304. Since typical disk media may have an internal diameter (ID)of 13 mm and an outer diameter (OD) of 30 mm, with at least 250 servospokes, the spatial distance between PES values along the tracks is:

(at ID): (π 13 mm)/250≅160 μm,

(at OD): (π 30 mm)/250≅380 μm.

The distance 324 corresponds to the spacing between track center 306 andthe negative PES limit 314 for track 304. Thus a WI event will only betriggered if track center 308 strays to the left of line 314. A benefitof the present invention is immediately apparent—the left-side WI limit314 for track 304 now takes into account the actual track center 306 forthe neighboring track 302 instead of merely employing the worst-casetrack center limit (i.e., negative limit 114 in FIG. 1). Clearly thismay allow closer spacing of tracks. Alternatively, other performancecriteria such as the number of write-inhibit (WI) events or the harderror (HE) rate may be improved. Various combinations of increasedtracks per inch and lower WI and/or HE events are also possible, as willbe illustrated in FIGS. 5 and 6, below.

FIG. 4 is a schematic graph 400 of the probabilities of various PESvalues 450 and 452 for two neighboring tracks in a data storage deviceconfigured according to the invention as illustrated in FIG. 3. Thedesired center 402 of track 1 corresponds to track 302 in FIG. 3, whilethe desired center 404 of track 2 corresponds to track 304. The verticalaxis in FIG. 4 corresponds to the probability of the PES having thevalues shown along the horizontal axis. The same assumptions about thenormal distribution of PES values apply as in FIG. 2. The same minimumallowable track center-to-center spacing W is also applicable here,since it is determined from characteristics of the write head (e.g.,writing width). The write inhibit criteria are (on a sector-by-sectorbasis):

PES(k)≧L=spacing 320 between track 304 and line 318, and

PES(k)≦−α L+PES(k−1)=spacing 316 between line 314 and track 304.

where k=the track number (either 1 or 2 in this example). Note that forthe worst-case PES scenario for track k−1 (i.e., where PES(k−1)=L), thenif α=2, we have the conventional case (on a sector-by-sector basis):

PES(k)≧L, and

PES(k)≦−α L+PES(k−1)=−2 L+L=L (with α=2 and for the worst-case ofPES(k−1)=L).

The parameter α is called the “FFWI Factor” and has a range of: 1≦α≦2.2in FIGS. 5-8. The optimum value for α is determined by the type ofregion being written (E- or I-) and the desired trade-offs between WIrate, HE rate, and TPI. FIGS. 5-8 discuss the range of choices for α andhow it affects these three operating parameters.

One implementation cost for the FFWI method is the requirement tomaintain in memory all the PES samples (typically 16-bit=2-byte each)for the last written track in each I-region. For a 250 sector track,this would require 500 bytes. Another implementation cost is the need toimpose sector-by-sector WI criteria which are different on the left- andright-hand sides of the track being written. PES information during anemergency power off (EPO) event may be retained, or, alternatively, amethod may be implemented for completing a first write into an I-regionwithout the PES data from the previous track (in this case, PES(k−1)=Lcould be assumed in the WI limit formula above, where k−1=the previoustrack for which we do not have actual PES data). One method foraccomplishing this would be to leave one physical track guard band forthe first I-region write operation just following an EPO event. In asequential bypass mode, where a new track is written into the middle ofthe I-region, the same strategy of leaving a physical guard band mightwork as well.

A potential problem with the FFWI method of the present invention is thepossibility of accumulating radial asymmetries as more and more tracksare written radially outwards (or inwards) in shingle mode. This may beameliorated by occasionally doing a “restart” with a track guard bandand then a track written using fixed WI limits (i.e., ±L) as discussedin FIGS. 1 and 2.

Data Storage Device Operation for E-Regions (Temporary Caches)

FIG. 5 is a graph 500 characterizing data storage device operationaccording to the invention for writing in a temporary cache region.System performance is compared between Shingled Mode Recording (SMR)without feed-forward (FF) [“SMR w/o FF”] and SMR with FF [“SMR w FF”],corresponding to the present invention. SMR w/o FF corresponds to PESlimits which are independent of the FFWI Factor α, which is plottedalong the horizontal axes 502, 602, 702, and 802, in graphs 500 through800, respectively. Thus, all curves for SMR without FF will behorizontal lines, independent of α. Conversely, all curves for thepresent invention will be functions of α. As discussed in the Backgroundsection, for temporary cache regions (“E-regions”), it is generally moreuseful to maximize writing speeds or “performance”, possibly at theexpense of data storage density (TPI). “Performance” may be decreased bytwo parameters: the hard-error (HE) rate, and the percent ofwrite-inhibit (WI) hits. Curve 508 corresponds to the probability of ahard error for SMR without FF, while curve 510 corresponds to theprobability of a hard error for the present invention—curves 508 and 510relate to axis 504. Curve 512 corresponds to the percentage of WI-hitsfor SMR without FF, while curve 514 corresponds to the percentage of WIhits for the present invention—curves 512 and 514 relate to axis 506.

For both axes 504 and 506, improved performance corresponds to lowervalues. Circle 530 is the intersection between curves 508 and 510—valueson curve 510 to the left of circle 530 correspond to values of the FFWIFactor for which the present invention (SMR w FF) enables lower harderror rates than for SMR without FF. Values on curve 510 to the right ofcircle 530 correspond to situations where SMR without FF has lower HErates making these FFWI factors undesirable. Circle 532 is theintersection between curves 512 and 514—values on curve 514 to the rightof circle 532 correspond to values of the FFWI Factor for which thepresent invention (SMR w FF) enables fewer WI-hits than for SMR withoutFF. Values on curve 514 to the left of circle 532 correspond to valuesof the FFWI Factor for which SMR without FF has lower WI-hits makingthese FFWI Factors also undesirable. The range of FFWI Factors betweencircles 532 and 530 correspond to a range over which the presentinvention is advantageous over SMR without FF with respect to both HErates and WI-hits. Arrow 516 (at α≈1.37) represents a 10.4 times lowerHE rate for the present invention with the same level of WI hits andsqueeze (see FIG. 6)—this may be beneficial under extreme operationalvibration conditions (see FIGS. 7 and 8). Arrow 518 (at α≈1.66)represents a 37.1% reduction in WI hits for the present invention—thisis more suited to an E-region since it enables higher writing speedswithout increasing the HE rate or write-to-write track misregistration(WWTMR). Thus values of the FFWI Factor over the range 1.37≦α≦1.66provide a range of choices providing both decreased HE rates anddecreased WI hits relative to the SMR without FF.

Data Storage Device Operation for I-Regions (Final “Home” or DestinationRegions)

FIG. 6 is a graph 600 characterizing data storage device operationaccording to the invention for writing in a final “home” or destinationregion. As in FIG. 5, system performance is compared between ShingledMode Recording (SMR) without feed-forward (FF) and SMR with FF,corresponding to the present invention. Curves 608 and 612 for SMRwithout FF are horizontal lines, independent of α. Curves 610 and 614for the present invention are functions of cc (horizontal axis 602). Asdiscussed in the Background section, for final “home” or destinationregions (“I-regions”), it is generally more useful to maximize datastorage density (TPI), possibly at the expense of writing speeds (whichmay be limited by WI hit rates and/or HE rates). Curve 608 correspondsto the “squeeze” at a given HE rate for SMR without FF, while curve 610corresponds to the probability of a hard error for the presentinvention—curves 608 and 610 relate to axis 604. Curve 612 correspondsto the percentage of WI-hits for SMR without FF, while curve 614corresponds to the percentage of WI hits for the presentinvention—curves 612 and 614 relate to axis 606 and represent the samedata as curves 512 and 514, respectively, in FIG. 5.

For both axes 604 and 606, improved performance corresponds to lowervalues. Circle 630 is the intersection between curves 608 and 610—valueson curve 610 to the left of circle 630 correspond to values of the FFWIFactor α for which the present invention enables lower squeeze values(more tracks per inch) than for SMR without FF. Values on curve 610 tothe right of circle 630 correspond to situations where SMR without FFhas lower squeeze making these FFWI factors undesirable. Circle 632 isthe intersection between curves 612 and 614—values on curve 614 to theright of circle 632 correspond to values of the FFWI Factor α for whichthe present invention enables lower WI-hits than for SMR without FF.Values on curve 614 to the left of circle 632 correspond to values ofthe FFWI Factor for which SMR without FF has fewer WI-hits making theseFFWI Factors undesirable. The range of FFWI Factors α between circles632 and 630 corresponds to a range over which the present invention isadvantageous over SMR without FF with respect to both squeeze andWI-hits. Arrow 616 (at α≈1.37) represents a 2.84% higher TPI for thepresent invention—this increases the areal density without degradingdata reliability. Arrow 618 (at α≈1.66) represents a 37.1% improvementin WI hits for the present invention with the same squeeze and HE rate.Thus values of the FFWI Factor over the range 1.37≦α≦1.66 provide amixture of higher TPI and decreased WI hits (without increasing the HErate or WWTMR) relative to SMR without FF.

Selection of FFWI Factor for Two Embodiments of the Present Invention

In a first embodiment of the present invention, the FFWI Factor α mayhave a different value for different physical zones on the storagemedium. As discussed above, for E-regions, the trade-off is betweenperformance and HE rates. This is applicable to any cache or temporarystorage region where data is first written, typically as fast aspossible to avoid delaying the host device. Tuning of the optimal αvalue may easily be performed on-line. For I-regions, the trade-off isbetween performance and higher TPI (denser storage). This is applicableto any region where data is finally stored and efficiency of storage ismore useful (maximize TPI). Although on-line adjustment is possible,off-line tuning may be most practical.

In a second embodiment of the present invention, in an operationalvibration (“op-vib”) environment, a trade-off between performance and HErates (similar to that used in writing E-regions) is applicable. As PESprediction rates are affected by op-vib, the FFWI Factor α may beadjusted to lower HE rates, or at least maintain HE rates in the face ofincreased vibration-induced head motions.

Data Storage Device Operation for E-Regions in Operational VibrationConditions

FIG. 7 is a graph 700 similar to graph 500 in FIG. 5, except under largespeaker vibration environment in a laptop computer while playing loudmusic. As a result of this op-vib environment, the ranges for the FFWIFactor α are shifted to lower values, typically over the approximaterange 1.2≦α≦1.56 (compared with the range 1.37≦α≦1.66 in the absence ofloud noise). This shift to lower α values compensates for the increasedhead motion induced by the op-vib environment since lower α values causethe FFWI method of the present invention to trigger WI events forsmaller negative head excursions (i.e., line 314 is moved to the rightin FIG. 3).

In FIG. 7, system performance is compared between Shingled ModeRecording (SMR) without feed-forward (FF) and SMR with FF, correspondingto the present invention. Curve 708 corresponds to the probability of ahard error for SMR without FF, while curve 710 corresponds to theprobability of a hard error for the present invention—curves 708 and 710relate to axis 704. Curve 712 corresponds to the percentage of WI-hitsfor SMR without FF, while curve 714 corresponds to the percentage of WIhits for the present invention—curves 712 and 714 relate to axis 706.

For both axes 704 and 706, improved performance corresponds to lowervalues. Circle 730 is the intersection between curves 708 and 710, whilecircle 532 is the intersection between curves 712 and 714. The sameconsiderations apply here as for FIG. 5. The range of FFWI Factors (axis702) between circles 732 and 730 corresponds to a range over which thepresent invention is advantageous over SMR without FF with respect toboth HE rates and WI-hits. Arrow 716 (at α≈1.2) represents a 2.88 timeslower HE rate with the same WI rate and squeeze—this may be beneficialunder extreme operational vibration conditions. Arrow 718 (at α≈1.56)represents a 32.2% improvement in WI hits with the same HE rate andsqueeze—this is more suited to an E-region since it enables higherwriting speeds without increasing the HE rate or write-to-write trackmisregistration (WWTMR). Thus values of the FFWI Factor α over the range1.2≦α≦1.56 provide a mixture of both decreased HE rates and decreased WIhits relative to SMR without FF.

Data Storage Device Operation for I-Regions in Operational VibrationConditions

FIG. 8 is a graph 800 similar to graph 600 in FIG. 6, except under largespeaker vibration environment in a laptop computer while playing loudmusic. As in FIG. 7, a result of the op-vib environment is a lower rangefor the FFWI Factor α. Curves 808 and 812 for SMR without FF arehorizontal lines, independent of α. Curves 810 and 814 for the presentinvention are functions of α. Curve 808 corresponds to the “squeeze” ata given HE rate for SMR without FF, while curve 810 corresponds tosqueeze for the present invention—curves 808 and 810 relate to axis 804.Curve 812 corresponds to the percentage of WI-hits for SMR without FF,while curve 814 corresponds to the percentage of WI hits for the presentinvention—curves 812 and 814 relate to axis 806 and represent the samedata as curves 712 and 714, respectively, in FIG. 7).

For both axes 804 and 806, improved performance corresponds to lowervalues. Circle 830 is the intersection between curves 808 and 810, whilecircle 832 is the intersection between curves 812 and 814. The sameconsiderations apply here as for FIG. 7. The range of FFWI Factors αbetween circles 832 and 830 corresponds to a range over which thepresent invention is advantageous over SMR without FF with respect toboth squeeze and WI-hits. Arrow 816 (at α≈1.2) represents the 2.55%higher TPI with the same WI and HE rates—this increases the arealdensity without degrading data reliability. Arrow 818 (at α≈1.56)represents a 32.2% improvement in WI hits with the same squeeze and HErate. Thus values of the FFWI Factor α over the range 1.2≦α≦1.56 providea mixture of higher TPI and decreased WI hits (without increasing the HErate or WWTMR).

Radial Distance Measurements: Non-Circular and Virtual Tracks, Effectsand Corrections

FIG. 9 is a schematic diagram 900 of two neighboring tracks, k−1 and k,in a data storage device employing SMR without FF, showing data squeeze.Track center 904 corresponds to track k−1 and track center 908corresponds to track k—individual PES values along track centers 904 and908 are illustrated by black circles 910. The radial spacing δ betweentrack centers 904 and 908 may not be representative of the actualinter-track spacing due to the trigonometric effect shown by thetriangle having a hypotenuse δ and angle θ. The tangent 902 to curve 904and the tangent 906 to curve 908 represent the local angles between thetrack center curves and the average track position. The actual distancebetween tangents 902 and 906 is δ′=δ sin(θ)≦δ, where θ may differ from90° by as much as 1° or more.

FIG. 10 is a schematic diagram 1000 of two neighboring tracks, k−1 andk, in a data storage device according to the invention, showing aproposed solution to the data squeeze problem illustrated in FIG. 9.Track center 1004 corresponds to track k−1 and track center 1008corresponds to track k—individual PES values along track centers 1004and 1008 are illustrated by black circles 1010. The tangent 1002 tocurve 1004 and the tangent 1006 to curve 1008 represent the local anglesbetween the track center curves and the average track position. Now,according to the present invention, the actual inter-track spacing isthe desired minimum value δ, while the radial spacing is Δ=δ/sin(θ)≧δ.Thus tracks 1004 and 1008 are no longer squeezed since the effectivetrack separation has been increased by the factor: 1/sin(θ)≧1.Implementation of this method for prevention of squeeze utilizes theservo PES values from multiple sectors on the previous track. Apolynomial fit is then made to these PES values to eliminate numericalerrors in the determination of the angle θ. The desired spacingΔ=δ/sin(θ) is then calculated, given the desired minimum spacing δ.

Servo Tracks, Data Tracks, and PES Directional Issues

FIGS. 11 and 12 show the use of virtual tracks in two differentsituations: FIG. 11 illustrates a situation in which the data tracks arenon-circular while the written servo tracks are circular, while FIG. 12illustrates the converse situation where the written servo tracks arenon-circular while the data tracks are circular. Actual situations maybe somewhere between the limiting cases illustrated in FIGS. 11 and 12,i.e., both the servo and data tracks may be non-circular (but stilloverlapping). In all cases, we can only measure the servo PES, which isderived when a read head passes over a servo burst region within a servospoke on the disk. This measured PES is always orthogonal to the servotracks (whether they are circular or non-circular) since it is derivedby the read head position relative to the servo burst pattern, which hasfeatures which are themselves perpendicular to the servo track.Unfortunately, the information which is needed for writing is thepositional error orthogonal to the data tracks, not the servo tracks.Examination of FIGS. 11 and 12 confirms that the servo and data tracksare not parallel to each other (and may typically even cross over), thusthe PES signal is essentially in the wrong direction (i.e., notorthogonal to the data tracks). The discussions below for FIGS. 11 and12 discuss the two scenarios of comparison between servo and datatracks.

When the servo data is pre-written (as may be the case for pre-patternedmedia), the servo tracks will generally be elliptical, since it isimpossible to define the center of rotation exactly enough to correspondto the center of patterning—this corresponds to a large 1× harmonicoffset of the servo tracks relative to the center of rotation. When theservo information is written by the write head in situ duringmanufacturing, the servo information will generally be approximatelycircular.

There are two general ways known for writing servo tracks:

External (Pre-written)—in this case, the servo tracks (which are definedby a multiplicity of servo burst patterns around the circumference ofeach track and located within the servo spokes), are pre-written duringpatterning of the disk medium before it is mounted within the disk drivebeing manufactured. Virtual data tracks may be used in this case to pushthe data tracks closer together than the servo tracks to increase TPI.In this example, the servo tracks are essentially always elliptical or1× (and possibly also 2×, 3×, etc.) harmonic offset from the center ofrotation.

Internal—in this case, the servo tracks are written by the write headafter the disk medium has been mounted within the disk drive beingmanufactured. Again, the data tracks may be pushed closer together thanthe servo tracks to increase TPI. In this case, the servo tracks may bemore precise relative to the center of rotation.

The servo tracks essentially define a “coordinate system” for writingthe data tracks, typically with 1.5 to 2 or more data tracks writtenradially for each servo track. If the servo tracks are not evenly spaced(i.e., are squeezed in some places) radially, there is no way for thesystem to detect this, and those data tracks written based on this servotrack “coordinate system” will also be squeezed as a result—this mayresult in undesirable excessive read errors. Virtual tracks allow thedata track spacings to be more uniformly spaced, thus reducing oreliminating excessive squeeze of the data tracks, thereby reducing readerrors. Data track squeeze can be reduced through the use of WIprotection according to the invention, however, since servo tracksqueeze is undetectable, its effects can only be reduced throughnon-circular and/or intersecting data tracks as shown in FIGS. 11 and12, below.

Non-Circular Data Tracks with Circular Servo Tracks

FIG. 11 is a schematic diagram 1100 of non-circular virtual data tracks1104 and circular servo tracks 1102. In general, there will typically be1.5 to 1.0 or more data tracks 1104 for each servo track 1102 (althoughthe example shown here shows a 1:1 ratio of track types). This situationwould most often be the case with internally-written servo burstinformation. The non-circular data tracks 1104 each may intersect one ormore of the circular servo tracks 1102. As is always the case, themeasured servo PES direction is orthogonal to local direction of theservo tracks, as discussed above. Each PES value is illustrated by ablack circle with attached arrows 1110 indicating the purely radialdirection of the positional error signals—the PES measurements in thisexample are purely radial because the servo tracks 1102 are circular.Arrows 1112 correspond to the direction of positional error that weactually want: orthogonal to the data tracks 1104, not to the servotracks 1102. A full rotation of the disk medium is indicated bydimension arrow 1108—data tracks 1104 pass through exactly one fullsinusoidal cycle across dimension 1108, consistent with a 1× harmonicdistortion.

Circular Data Tracks with Non-Circular Servo Tracks

FIG. 12 is a schematic diagram 1200 of circular virtual data tracks 1204according to the invention, each corresponding to a non-circular servotrack 1202. As in FIG. 11, there will typically be 1.5 to 1.0 or moredata tracks 1204 for each servo track 1202 (although the example shownhere shows a 1:1 ratio of track types). This situation would most oftenbe the case with externally-written servo burst information. Thecircular data tracks 1204 each may intersect one or more of thenon-circular servo tracks 1202. As is always the case, the measuredservo PES direction is orthogonal to the servo tracks, as discussedabove. Each PES value is illustrated by a black circle with attachedarrows 1210 indicating the typically non-radial directions of thepositional error signals—the PES measurements in this example areusually non-radial because the local slopes of the servo tracks 1202 areusually non-zero (except the small number of sectors where the servotrack is turning around radially). Arrows 1212 correspond to the radialdirections of positional error that we actually want: orthogonal to thedata tracks 1204. A full rotation of the disk medium is indicated bydimension arrow 1208—servo tracks 1202 pass through exactly one fullsinusoidal cycle across dimension 1208, consistent with a 1× harmonicdistortion.

Use of Virtual Tracks to Reduce Data Track Squeeze

FIG. 13 is a schematic diagram 1300 of two neighboring virtual tracks1304 and 1314, illustrating smooth (i.e., low spatial frequency)polynomial or harmonic fits to the PES measurements. The conventionalcircular tracks 1302 and 1312 show how the virtual tracks 1304 and 1313oscillate radially inwards and outwards relative to tracks 1302 and1312, respectively, with an overall average deviation of zero. PESvalues along the track centers 1306 and 1316 are illustrated by blackcircles 1330, with the corresponding PES error information always beingorthogonal to the local slope of the servo track. The calculation of therequired radial track spacing Δ employs a θ value which is determinedusing the smoothed virtual tracks to reduce numerical errors. Thisdiagram applies to FIG. 11, where the servo tracks are circular and thedata tracks are non-circular. The polynomial curves 1306 and 1316provide high spatial frequency (small magnitude) positional information,while the corresponding underlying virtual tracks 1304 and 1314 providelow spatial frequency (large magnitude) positional error data. Tracks1304 and 1314 would be used to generate local values of θ (to avoid therapid variations in data which could occur if polynomial curves 1306 and1316 were used to generate θ). The polynomial curves 1306 and 1316 areused to generate the local values for Δ, as shown.

Flowchart for Implementing Radial Distance Corrections

FIG. 14 is a flowchart 1400 for implementing a feed forward writeinhibit (FFWI) method according to the present invention. In block 1402,position error signals (PES) are acquired during the writing of thecurrent track as the read head passes across servo burst regions. Asdiscussed above, there are typically around 250 servo spokes or zones,each containing a servo burst region used to generate the a PES valuefor each rotation of the disk medium (giving a total number of PESvalues equal to the number of servo spokes). Each PES value maytypically comprise 2 bytes of radial position information. As discussedbelow in FIG. 15, this data may be stored in a cache memory and/or onthe disk itself. An advantage of storage on the disk is that the datawill then be non-volatile and thus still available for use duringrecovery from an emergency power off (EPO) event. Next, in block 1404,the localized deviation of the data track with respect to the servotracks is computed. A harmonic (i.e., 1×, 233 , etc.) sinusoidalwaveform relative to the disk rotation (e.g. curve 1304) or a polynomialfit of the PES servo data (e.g., curve 1306) enables the determinationof the local slope of the servo track for each sector around the track.Block 1406 then computes the required radial head position correctionbased on angle and squeeze requirements, as discussed in FIG. 13. Thiscorrection is now more accurate since it is based on the multiple servosector PES (computed in block 1404), instead of just the PES at a singlesector. Finally, in block 1408, the required radial correction fromblock 1408 is applied to either the write inhibit (WI) limit or is usedas the reference trajectory during writing of subsequent tracks.

Use of the Feed-Forward Write Inhibit Method with Virtual Tracks

The virtual track concept may be utilized in conjunction with thefeed-forward write inhibit method. In this case, the benefits ofimproved storage densities will enable the data tracks (either circularor non-circular) to be written more closely together using the“coordinate system” defined by the overlapping servo tracks (eithernon-circular or circular). In addition, improved writing performance, isalso possible using virtual tracks.

Alternatively, the virtual track concept may be utilized in the absenceof the FFWI method. In this case, the upper write-inhibit limit would beset to:

PES(k)≧L, and

PES(k)≦−L,

as discussed in the Background section, instead of to:

PES(k)≧L, and

PES(k)≦−α L+PES(k−1),

where α=the FFWI factor.

Data Storage System Embodying the Present Invention

FIG. 15 is a schematic diagram of a data storage system 1500 embodyingthe present invention. System 1500 includes a host computer 1502, astorage device 1510, such as a hard disk drive 1510, and an interface1534 between the host computer 1502 and the storage device 1510. Hostcomputer 1502 includes a processor 1504, a host operating system 1508,and control code 1506. The storage device or hard disk drive 1534includes a controller 1514 coupled to a data channel 1520. The storagedevice or hard disk drive 1510 includes an arm 1528 carrying aread/write head including a read element 1524, and a write element 1526.The hard disk drive (HDD) 1510 advantageously is a Shingled Disk Drive(SDD) to achieve high track density recording magnetic patterns of dataon a writable disk surface 1522 of disk 1532 in overlapping circulartracks using shingled magnetic recording (SMR).

In operation, host operating system 1508 in host computer 1502 sendscommands to hard disk drive 1510. In response to these commands, harddisk drive 1510 performs requested functions such as reading, writing,and erasing data, on disk surface 1522. Controller circuit 1514 causeswrite element 1526 to record magnetic patterns of data on a writablesurface of disk 1522 in tracks 1530. The controller circuit 1514positions the read head 1524 and write head 1526 over the recordable orwritable surface 1522 of a disk 1532 by locking a servo loop topredetermined servo positioning burst patterns, typically located in aservo spokes or zones. The predetermined servo positioning pattern mayinclude a preamble field, a servo sync-mark (SSM) field, a track/sectoridentification (ID) field, a plurality of position error signal (PES)fields, and a plurality of repeatable run out (RRO) fields following theburst fields.

In accordance with embodiments of the invention, system 1500 includes acache memory 1512, for example, implemented with one or a more of: aflash memory, a dynamic random access memory (DRAM) and a static randomaccess memory (SRAM). A sensor 1516, such as an accelerometer, detectsoperational vibration conditions and provides operational vibrationdisturbance spectrum information to the controller 1514. An adjustedtrack pitch information table 1518 stores changed track pitchinformation when an operational vibration condition occurs.

In accordance with embodiments of the invention, controller circuit 1514saves the PES information 1518 for a full rotation of disk 1532 for useduring read operations. This PES data 1518 can be written to specificlocation on the disk to retain the information during a power off event.

System 1500 including the host computer 1502 and the storage device orhard disk drive 1510 is shown in simplified form sufficient forunderstanding the present invention. The illustrated host computer 1502together with the storage device or hard disk drive 1510 is not intendedto imply architectural or functional limitations. The present inventioncan be used with various hardware implementations and systems andvarious other internal hardware devices.

Alternative Embodiments within the Scope of the Present Invention

Although the present invention has been described in the context of harddisk drives, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

1. A data storage device comprising: a rotating data storage medium,comprising: data tracks; servo data sectors, comprising servo burstpatterns; a read/write head assembly, comprising a read head configuredto read data from the rotating data storage medium, and a write headconfigured to write data to the rotating data storage medium; and acontroller for controlling the position of the read/write head assemblyrelative to the rotating data storage medium, wherein the controller isconfigured to: receive positional error signals from the read headgenerated when the read head passes over a servo burst pattern; storethe positional error signals generated from all servo burst patternsaround a track on the rotating data storage medium as data is beingwritten by the write head to the rotating data storage medium; calculatea fixed first write inhibit limit on a first side of the track beingwritten, and a variable second write inhibit limit on the side of thetrack being written opposite from the first side; and determine whetherthe positional error signal is between by the two write inhibit limits,wherein: if the positional error signal is between the first and secondwrite inhibit limits, then enable writing on the rotating data storagemedium; or if the positional error signal is not between the first andsecond write inhibit limits, then inhibit writing on the rotating datastorage medium.
 2. The data storage device as in claim 1, wherein thevariable second write inhibit limit comprises a component proportionalto the fixed first write inhibit limit.
 3. The data storage device as inclaim 2, wherein the proportionality constant preferably ranges fromminus 1.0 to minus 2.2.
 4. The data storage device as in claim 2,wherein the proportionality constant more preferably ranges from minus1.2 to minus 1.7.
 5. The data storage device as in claim 1, wherein thesecond write inhibit limit comprises a component proportional to thepositional error signal of a neighboring track.
 6. The data storagedevice as in claim 5, where the constant of proportionality is one. 7.The data storage device as in claim 1, wherein the controller is furtherconfigured to fit a function to the sequence of positional error signalsaround the track.
 8. The data storage device as in claim 7, wherein thefunction is a polynomial.
 9. The data storage device as in claim 7,wherein the function is a harmonic of a single rotation of the rotatingdata storage medium.
 10. The data storage device as in claim 9, whereinthe harmonic is a first-harmonic of a single rotation of the rotatingdata storage medium.
 11. The data storage device as in claim 7, whereinthe controller is further configured to compute the derivative of thefunction with respect to rotation of the rotating data storage medium.12. The data storage device as in claim 11, wherein the controller isfurther configured to compute a required radial correction to the trackposition based on the derivative of the function and a predeterminedminimum track spacing.
 13. The data storage device as in claim 12,wherein the controller is further configured to adjust the first andsecond write inhibit limits based on the required radial correction tothe track position.
 13. A method for varying write inhibit limits in adata storage device comprising: a rotating data storage medium,comprising: data tracks; servo data sectors, comprising servo burstpatterns; a read/write head assembly, comprising a read head configuredto read data from the rotating data storage medium, and a write headconfigured to write data to the rotating data storage medium; and acontroller for controlling the position of the read/write head assemblyrelative to the rotating data storage medium, wherein the controller isconfigured to receive positional error signals from the read headgenerated when the read head passes over a servo burst pattern; themethod comprising: writing a first data track on the rotating datastorage medium subject to write inhibit limitations based onpredetermined fixed write inhibit limits on both sides of the first datatrack; saving the positional error signals generated during writing ofthe first data track; and writing a second data track, in proximity tothe first data track, subject to write inhibit limitations based on afixed first write inhibit limit on the side of the second data trackopposite from the first data track, and a variable second write inhibitlimit on the side of the second data track adjacent to the first datatrack; wherein if the positional error signal is between the first andsecond write inhibit limits, then enable writing of the second datatrack; or if the positional error signal is not between the first andsecond write inhibit limits, then inhibit writing of the second datatrack.
 14. The method as in claim 13, wherein the variable second writeinhibit limit comprises a component proportional to the fixed firstwrite inhibit limit.
 15. The method as in claim 14, wherein theproportionality constant preferably ranges from minus 1.0 to minus 2.2.16. The method as in claim 14, wherein the data storage device furthercomprises a vibration sensor, configured to convey a vibration levelsignal to the controller; and wherein the proportionality constant has afirst value when the vibration level indicates non-vibrational andnon-shock operating conditions, and the proportionality constant has asecond value when the vibration level signal indicates vibrational orshock operating conditions.
 17. The method as in claim 14, wherein theproportionality constant has a first value when writing temporary cacheregions of the rotating data storage medium, and the proportionalityconstant has a second value when writing final home or destinationregions.
 18. The method as in claim 13, wherein the variable secondwrite inhibit limit comprises a component proportional to the positionalerror signal of the first track.
 19. The method as in claim 18, wherethe constant of proportionality is one.
 20. The method as in claim 13,wherein a function is fitted to the sequence of positional error signalsaround the track.
 21. The method as in claim 20, wherein the function isa polynomial.
 22. The method as in claim 20, wherein the function is aharmonic of a single rotation of the rotating data storage medium. 23.The method as in claim 22, wherein the harmonic is a first-harmonic of asingle rotation of the rotating data storage medium.
 24. The method asin claim 20, wherein the derivative of the function with respect torotation of the rotating data storage medium is computed.
 25. The methodas in claim 24, wherein a required radial correction to the trackposition is computed, based on the derivative of the function and apredetermined minimum track spacing.
 26. The method as in claim 25,wherein the first and second write inhibit limits are adjusted based onthe required radial correction to the track position.
 27. A method fordefining virtual data tracks in a data storage device comprising arotating data storage medium, comprising: defining a multiplicity ofservo tracks on the rotating data storage medium; determining a desiredpattern of data tracks to be written on the data storage medium, thedesired pattern comprising the radial spacings of the data tracks, andthe degree of circularity of the data tracks; defining a coordinatesystem for the data tracks based on the positional error signals of themultiplicity of servo tracks.
 28. The method of claim 27, wherein theservo tracks are substantially circular and the data tracks aresubstantially non-circular.
 29. The method of claim 27, wherein theservo tracks are substantially non-circular and the data tracks aresubstantially circular.
 30. The method of claim 27, wherein both theservo tracks and data tracks are substantially non-circular.
 31. Themethod of claim 27, wherein the servo tracks and data tracks overlapcircumferentially around the rotating data storage medium.